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2 Vibrations What is a vibration in a molecule?Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bondsCan a vibration change the dipole moment of a molecule? (Shockwave animation)Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active.Symmetrical stretching/bending does not. Not IR active

3 120 trillion vibration per second!!!!What wavelength of electromagnetic radiation is involved in causing vibrations in molecules?Infrared (IR) electromagnetic radiation causes vibrations in molecules (wavelengths of ,000 nm or 2.5 – 15 mm)For a vibration at 4111 cm-1 (the stretch in H2), how many vibrations occur in a second?120 trillion vibration per second!!!!120 x 1012 vibrations/sec or a vibration every 8 x seconds!WOW !!!!!

4 How does the mass influence the vibration?MM =2 g/moleMM =254 g/moleThe greater the mass - the lower the wavenumber

5 How much movement occurs in the vibration of a C-C bond?stretching vibration10 pm154 pmFor a C-C bond with a bond length of 154 pm, the variation is about 10 pm.bending vibrationFor C-C-C bond angle a change of 4o is typical.This moves a carbon atom about 10 pm.4o10 pm

6 A little physics of electromagnetic radiation= hcn’Energy (E) E = hn = hc/lwhere h is Planck’s constant, c is the speed of light, n is frequency or the number of vibrations per second and l is the wavelengthWavenumber (n’) n’ = 1/ lgiven in cm-1Period (P) P = 1/nthe time between a vibrationEnergy, frequency, and wavenumber are directly proportional to each other.

7 What type of vibrations would occur in pentane?Let’s examine the IR spectrum of pentane.

11 Does the stretching energy have any relationship to the strength of the bond?W = BEr2 =

12 Let’s examine the carbonyl group on three compoundsformaldehydephosgeneacetoneHow does the C=O stretching energy compare for these three molecules?2053 cm-11951 cm-12063 cm-1The carbonyl group has a range of cm-1.

13 Functional group analysis in organic compoundsUnlike atomic spectroscopy where sharp energy transitions occur due to well quantized electron transitions, molecular spectroscopy tends to show bands.Molecular vibrations are influenced by the surrounding groups!

15 Use of IR spectraIdentification of functional groups on a molecule – this is a very important tool in organic chemistrySpectral matching can be done by computer software and library spectraSince absorbance follows Beer’s Law, can do quantitative analysis